Many microtechnological applications and devices require high aspect ratio structures such as holes. One eminent example is Through-hole silicon via (TSV) used as interconnect in and between silicon chips. The making of high aspect ratio holes (harh) is in many cases difficult, expensive or even impossible. Limits of current hole making technologies such as laser ablation or etching are the minimum hole size, the aspect ratio and the roughness of the inner hole linings/walls. These limits currently pose a serious bottleneck to the large scale application of stacked silicon chips. The devices and methods described here provide a simple, inexpensive and precise approach to circumvent these limitations.
A further problem of current micromachining technologies is the specificity to selected materials. For instance, anisotropic etching of Si using KOH solutions is a simple way to produce grooves and holes in silicon but does not work with many other semiconductors or amorphous materials. Even with Si it functions only for specific crystal lattice orientations. It was therefore a goal of the here disclosed invention to provide a method that allows the machining of many different materials, most of them relevant to current microtechnology such as Si, Ge, GaAs, InP, Sapphire, glass, zirconia. Modifications to the method to machine different materials or even material classes were sought to be minor and easily implantable.
A further goal of the disclosed invention was the applicability of the method to the fabrication of arrays of holes. WO2005/097439 and PCT/EP2008/009419 disclose methods of generating structures in substrates using the application of voltages to a substrate. The methods disclosed therein do not allow a close spacing of holes due to the high voltages applied and the occurrence of voltage-flash-overs through already existing holes. Accordingly there is a need for methods and devices to prevent these detrimental effects for array formation.
Also, many miniaturized fluidic and chemical/biological analysis devices require small reservoirs and connection channels. The dimensions of these channels and containers are often in the micrometer range. Common micromachining techniques, developed mostly for planar structures, fall short of making wells and channels which enter deep into the chip/substrate. That is, the achievable aspect ratio—the ratio between the length and the diameter of a hole, is limited to typically 1:10. This is in particular true for the machining of glass and glass like materials such as fused silica. Significant drawbacks for large scale application derive also from the high production cost.
On the other hand, channels with very high aspect ratio allow for efficient electro-osmotic pumping of fluids through these channels, requiring e.g. for channels of 150 um length and 2 um diameter only small voltages and currents (e.g. 5 V) for significant fluid velocities within the channel. Very high aspect ratios will also allow to connect both sides of a typical glass chip of e.g. 0.5 mm thickness by trans-chip channels, thereby enabling simple three-dimensional fluid designs.
Channels with picoliter capacities will also provide a basis for picoliter fluidics, utilizing fluid transport and mixing effects irrelevant in larger volumes.
Accordingly it was an object of the present invention to provide for a method allowing the production of high quality perforated substrates. It was also an object of the present invention to provide for a method of production of such high quality membrane carriers which method is easy to perform and reproducible. It was furthermore an object to provide for a method allowing the controlled production of holes, cavities or channels in substrates, wherein the geometrical features of the holes, cavities and channels can be easily controlled and influenced. It was also an object of the present invention to provide for a method allowing the mass production of perforated substrates. It was furthermore an object of the present invention to provide a method of hole production that can be applied to substrates that were hitherto difficult to process, such as glass, sapphire or elemental silicon.